The field of electro-optics and nonlinear optics is concerned primarily with the use of light as a carrier of information. This information may take the form of visible images, digital data or analog signal information. In order to utilize light in this manner, it is necessary to develop media which will provide for the production, the transmission, the detection, and for the processing of the information impressed upon the light. One very important means for directing and processing light involves the use of waveguides. A waveguide is a physical structure which is capable of transmitting light (that is, it is transparent) while at the same time confining it to a particular region in space. In an optical waveguide, confinement of light may be achieved by varying the refractive index (an optical material property) within the region of the waveguide so that it is higher than that of its surroundings. Light will tend to propagate within the region of higher refractive index and so can be moved from a starting point to an ending point along a path which is defined by the waveguide region.
An important subclass of waveguides are nonlinear and electro-optically active waveguides. In these waveguides the light is not simply passively transported from one point to another, but is acted upon or processed while traveling within the guide. That is, these waveguides are capable of changing the nature of the light as it passes through. Important types of changes include rotation or alteration of the state of optical polarization, modulation of the amplitude of the optical intensity, modulation of the phase of the optical radiation, alteration of the directional characteristics of the radiation, and alteration of the frequency (or wavelength) of the radiation. By altering the properties of the radiation within a waveguide or a waveguide region, it is possible to encode and decode information and to route it as desired.
Electro-optically active waveguides have predominantly been fabricated from inorganic crystalline materials such as lithium niobate. These materials in general have high dielectric constants and moderate electro-optic coefficients. They are crystalline materials which require high temperature processing and thus are not easily fabricated and are not easily integrated with other semiconductor devices. Further, since the source of their electro-optic activity stems from motions of atoms in their crystal lattice, which is slow compared to optical frequencies, these materials are limited in their frequency response. These limitations have led to interest in using organic and especially polymeric organic materials for nonlinear and electro-optic applications. Organic materials have generally lower dielectric constants and certain organic materials have been shown to possess very large nonlinearities and electro-optic coefficients. Organic and polymeric organic materials are processed at much lower temperatures than inorganics and are amenable to solution and other coating techniques that make them much easier to fabricate than inorganic crystalline materials and which provides for processing conditions appropriate for integration with delicate semiconductor devices. Thus, organic materials are much more suitable for achieving the goals of integrated electro-optics, where light production, light processing and detection can all be accomplished in close proximity without the need for extraneous connections. In addition, the source of the optical nonlinearity of organic materials lies in the motion of their electrons which is much faster than the motion of a crystal lattice and thus, organic materials are of greater utility for processing information at high frequencies.
There is a need however for techniques and materials to provide for the fabrication of waveguides in thin or thick films of nonlinear and electro-optically active organic materials. There are several methods taught by the prior art for creating waveguiding regions in films of inactive polymeric films. U.S. Pat. No. 3,809,732 teaches a technique called "photo-locking" by which passive, inactive waveguides can be produced by a multistep process involving one of three types of chemical reactions between a transparent polymer and a high index, photoactive "doping" monomer. These three are chemical attachment, dimerization, and polymerization. In each case the polymer is doped with the monomer (which must be volatile), then photo-locked in the desired region and baked to remove the volatile monomer from all other areas. Brady has shown [M. Brady and P. Heldrich, IBM Technical Disclosure Bulletin, 23, 2999 (1980)] that polyimide organic films can be heated with a laser to thermally increase the index of refraction of selected areas. Tomlinson has shown [W. Tomlinson et al., Appl. Phys. Lett. 16, 486 (1970)] that photocrosslinking increases the local density of poly(methyl methacrylate) and thus increases its refractive index. Wells has shown [P. Wells and D. Bloor, in "Organic Materials for Non-linear Optics", [Royal Society of Chemistry, London, pg. 398 (1989)] that an organic dye dissolved in the polymer poly(4-vinyl pyridine) could be photochemically bleached by exposure to a mercury discharge lamp resulting in a lowered refractive index surrounding a waveguide region. Hallam has shown [A. Hallam et al., IEE Conf. Publ. 201, 26 (1981)] that photochromic fulgides materials dissolved in polymers can be converted from a colorless to a colored state, thus increasing the refractive index in a waveguide region. Weber has shown [H. Weber et al., Appl. Phys. Lett., 20, 143 (1972)] that photoresist can be patterned with an argon ion laser to produce waveguide structures.
A number of references teach the use of polymers which may be photochemically crosslinked to achieve free-standing "ribs" of polymer after the remaining, uncrosslinked, material is dissolved away H. Weber et al., Appl. Phys. Lett., 20, 143 (1972)]; N. Takato and T. Kurokawa, Appl. Opt., 21, 1940 (1982); B. Srinivasan and G. Martin, J. Appl. Polym. Sci., 29, 2231 (1984); and J. Giuliani et al., Appl. Phys. Lett., 48, 1311 (1986)]. Polymer ribs produced in this way may serve as waveguides. In each of these, the result is a passive waveguide structure. The more desirable fabrication of a nonlinear or electro-optically active waveguide structure is not achieved. Nonlinear or electro-optically active waveguides have been produced by means of lithography and etching on the substrates or on the organic material layers. Lytel has shown [R. Lytel, ACS Workshop, Organic and Polymeric Nonlinear Optical Materials, May 16-19, Virginia Beach, Va. (1988)] that a waveguide pattern can be produced in a metal film by lithography and etching. The metal then forms electrodes used to "pole" the organic material between them and thus change the refractive index along the electrode direction. Brettle has shown [J. Brettle et al., "Polymeric non-linear optical waveguides", Proc. SPIE 824, 171 (1987)] that etched metal electrode patterns can serve as masks for the diffusion of monomeric, nonlinear organic molecules into polycarbonate films by dipping into hot solutions of the monomers. Small has shown [R. D. Small et al., "Thin Film Processing of Polymers for Nonlinear Optics", Proc. SPIE 682, 160 (1986)] that etching a mask material on top of a layer of organic polymer followed by anisotropic etching can create waveguide structures in the polymer layer. In these examples, physically defined waveguide regions are obtained in nonlinear or electro-optically active organic materials by using lithography and etch techniques. These techniques suffer not only from increased fabrication complexity, but also from the fact that the substrate must be subjected to the harsh conditions of lithography and liquid or reactive ion etches. Since the goal of integrated optics is to fabricate optical processors and detectors in close proximity, these techniques may be largely unsuitable due to the damage that may result to sensitive electro-optic devices and related processors.
There is a need for a method which will produce nonlinear or electro-optically active waveguide structures, such as are possible in crystalline inorganic materials, in the more desirable organic thin films, but without utilizing techniques which may be deleterious to other substrate components.